This invention relates to a temperature measuring device for transformers and choke coils, particularly for those parts thereof which are subjected to a voltage, in which device the heat sensing element used is an oil-filled chamber, which is in good thermal contact with the part whose temperature is to be monitored, and from which at least one pipe, in the form of a rising pipe, passes through the lid of the transformer.
Of particular concern in the construction and operation of transformers is the mastering of the heating problems which govern the life of the oil and solid insulation, it being particularly desirable to be able to monitor the temperature of those parts of a winding which are particularly endangered from the thermal point of view by so-called hot spots. The main difficulty in connection with this temperature monitoring consists in that winding parts under high tension have to be thermally monitored, that is to say parts for which customary temperature sensing means in which the sensor is a resistance element cannot be used.
The methods described below have hitherto been applied for this purpose, one of them also being included in the patent literature (Austrian Patent Specification No. 256,505, German Patent Specification No. 1,270,835, Swiss Patent Specification No. 452,691, and a number of additional corresponding patents). The measuring sensing means comprises flat chambers of insulating material which are inserted into the cooling ducts of the winding with good thermal contact for the purpose of insulating the parts of the winding which are to be monitored, these ducts being in communication with the oil of the cooling duct and being connected by way of a flexible insulating tube to a chamber lying outside the high-tension zone and containing a thermocouple. At predetermined intervals of time the oil is drawn off from the chamber in question by means of a pump, this being effected so rapidly that the oil reaches the thermocoule practically without cooling, so that the temperature of the oil in the chamber can be measured. Another proposal consists in utilising the dependence on temperature of the resonant frequency of a quartz crystal, which is in good thermal contact with the winding part which is to be monitored, for controlling an RC oscillator, whose amplified output feeds a transducer radiating ultrasonic signals, which are received by a receiver provided on the wall of the transformer tank, and converted into an indication of temperature. According to a third proposal the temperature sensing means provided is a thermistor, which controls the output of an oscillator associated with it. This oscillator in turn modulates the frequency of another oscillator, which is controlled by a quartz crystal and whose output is radiated through an antenna. A receiving antenna mounted on the wall of the tank is connected to a frequency-current transformer whose output feeds a temperature indicator (Electrical Review, June 7, 1974, p. 647 to 650). Finally, in connection with the prior art mention may also be made of the direct measurement of the maximum oil temperature with the aid of a teflon tube provided at the top end of a transformer winding. This teflon tube is closed at one end, while its other end leads into a thinner teflon tube passing through the transformer tank. The entire teflon tube is filled with a predetermined volume of oil, which in consequence of its position in the uppermost part of the transformer tank participates in the temperature fluctuations of the hottest transformer oil. The resulting level fluctuations in the part of the teflon tube which passes through the transformer tank, or the resulting fluctuations in pressure at the end of the said tube, serve as an indication of the temperature of the oil in the uppermost part of the transformer tank and enable conclusions to be reached with regard to the thermal loading of the transformer (see CIGRE International Conference on Large High Tension Electric Systems), 1972 Session, Aug. 28-Sept. 6, Paper 12 -02 "Hot spot and top-oil temperatures. Proposal for a modified heat specification for oil immersed power transformers").
A temperature measuring device of the kind first defined above is known from German Patent Specification No. 526,732. In present-day technology, however, this temperature measuring device can no longer be used, particularly because the amount of oil required for this measuring process is very great, so that the dimensions of the oil tank would also have to be of corresponding size. However, with the gap width between coils customary at the present time, a large oil tank can no longer be installed. That a relatively large amount of oil is used in this method of measurement is clear from the fact that the amount of oil present in the oil tank must be far greater, than that present in the rising pipe, since otherwise the error which would be included in the result of the measurement because of the expansion of the oil in the rising pipe would no longer be acceptable.
The invention seeks to take into account the existence of small gap widths, and nevertheless to provide a reliable method of measurement. A temperature measuring device of the kind first defined above is characterized, according to the present invention, in that at least one additional oil-filled pipe, which at its end near the chamber is closed directly upstream of the chamber, is guided parallel to the aforesaid pipe, and that the oil taken out of the transformer tank by means of these two pipes constitutes the dielectric of two preferably cylindrical capacitors whose capacitances, which vary with the fluctuations of level of the dielectric, can be compared with one another and used to measure the temperature.
With the temperature measuring device of the invention it is possible, for the first time to enable the amount of oil in the oil tank to be designed independently of the amount of oil contained in the pipes, since the expansion of the oil in the pipe has no influence on the final result of the measurement. The expansion of the oil in the rising pipe is measured by the amount of oil contained in the second, parallel pipe and is deducted by the external measuring device.
Another advantage of the invention is to be seen in that, because of the small size of the oil tank the temperature pattern is not disturbed and the winding filling factor is not changed.